Scanning probe microscopes (SPM) are conventionally used to observe minute samples. Examples of a typical SPM include an atomic force microscope (AFM) and a scanning tunneling microscope (STM). An SPM brings a sharply pointed probe close to a sample, detects an interaction that acts between the probe and the sample, and performs feedback control with respect to the distance between the probe and sample so that the interaction is kept constant. The interaction is a tunnel current or an interactive force or the like. The SPM scans the probe (or sample) in a horizontal direction in a state in which feedback control is maintained. As a result, the probe (or sample) rises and falls so as to trace over the concavities and convexities of the probe (or sample). By recording the trajectory of the scan, a topographic image of the sample surface is obtained.
With an SPM, the aforementioned horizontal direction of scanning is referred to as the “XY direction”, and the vertical direction is referred to as the “Z direction”. A scanner of an SPM is configured to be able to perform scanning in the X direction, Y direction, and Z direction. For example, a conventional common tripod-type scanner has a configuration in which three scanners that are arranged on an X-axis, Y-axis, and Z-axis, respectively, are combined at one location. Each actuator comprises a piezoelectric body (piezo element). A tripod-type scanner is disclosed, for example, in Japanese Patent Laid-Open No. 63-265573.
In order to perform surface shape observation at a high speed using an SPM, it is necessary for a scanner to scan a probe (or sample) at a high speed. The scanning frequency of a scanner is limited by the resonance frequency in each axial direction. Accordingly, in order to perform high-speed imaging using an SPM, it is necessary to make the resonance frequency of the scanner as high as possible. When performing observation with an SPM, the scanner is required to operate at the fastest speed in the vertical (Z) axis direction. In many cases, the overall operating speed is limited by the resonance frequency in the Z-axis direction. It is therefore particularly important to increase the resonance frequency in the Z-axis direction.
As shown in FIG. 1A, the resonance frequency when a piezoelectric body of a Z-axis scanner oscillates freely in the vertical direction is taken as f0. A conventional common scanner has a structure in which one end is fixed, as shown in FIG. 1B. In this conventional structure, the bottom surface of the piezoelectric body is fixed to a support part. According to this structure, the resonance frequency in the Z direction is half of the resonance frequency f0 for free oscillation, that is (½) f0, and consequently the resonance frequency is significantly deceased.
According to the conventional structure shown in FIG. 1B, a displacement of the piezoelectric body of the Z-axis scanner oscillates asymmetrically with respect to the center of gravity of the piezoelectric body. Consequently, a large impact is generated by the oscillation. This impact is transmitted through the support part on the lower side, and increases a force that excites a resonant oscillation of other portions that are included in the X-axis and Y-axis scanners. When oscillation of another portion is excited, conversely the impact of such oscillation is transmitted in the Z-axis direction, leading to the occurrence of an oscillation in the Z-axis direction. As a result, it is not possible to use the scanner at a frequency that is greater than or equal to the resonance frequency (lower than the resonance frequency in the Z-axis direction) of other portions included in the scanner. This phenomenon further limits the scanning frequency of the Z-axis scanner to a frequency lower than the resonance frequency.
The resonance frequency of free oscillation of a Z-axis scanner is, for example, approximately several hundred kHz. Further, an oscillation frequency caused by the above described impact is, for example, several tens of kHz. On the other hand, the scanning frequency of a conventional common scanner is, at the highest, approximately 1 kHz. Therefore, in the conventional common scanners, the resonance frequency or oscillations caused by an impact have not constituted a significant problem. However, the resonance frequency and oscillations caused by an impact become a problem when attempting to increase the scanning frequency to increase the speed of an SPM.
Japanese Patent Laid-Open No. 2002-82036 proposes supporting a center part of a side surface of a piezoelectric body. This structure can decrease the oscillations of a scanner by balancing impacts that arise in the upper half and lower half of the piezoelectric body. However, since one side surface of the piezoelectric body is being supported, another mode of oscillation arises, and this fact limits the scanning frequency of the scanner. Further, the structure that supports a side surface of a piezoelectric body reduces the resonance frequency in comparison with a free oscillation state.